Cladding of strip material



May 7, 1968 B. v. WREN ETAL CLADDING OF STRIP MATERIAL Filed Jan. l5, 1964 WUQQQW United States Patent 4O 3,382,085 CLADDING OF STRIP MATERIAL Brian Vickerson Wren, Middlesbrough, Clillord Barratt, Stockton-on-Tees, Peter David Swales, Guisborough, and Kenneth Thomas Lawson, Middlesbrough, England, assignors to Head, Wrightsou and Company, Limited, Thornaby-on-Tees, England, a corporation of the United Kingdom l Filed Jan. 15, 1964, Ser. No. 337,895 Claims priority, application Great Britain, Jan. 17, 1963, 2,184/ 63 11 Claims. (Cl. 117-17) ABSTRACT F THE DISCLOSURE A method of coating a strip formed from any one or combination of a large number of metals with a protective surface of a differing metal which also may be any one of or combination of a large number of metals. The strip is wetted by immersion in a bath `containing any of a number of liquids, preferably Iof relatively low surface tension. A powder containing electrostatically charged particles of metal differing from the metal of the strip is applied to the wetted strip. The metal strip and metallic powder are dried and the powder is compacted against the surface of the strip to provide the aforesaid coating. Subsequent heating of the metal strip and coating combination provides bonding of the coating to the strip.

This invention relates to an improved method of coating metal strip with a protective surface of another metal, when applied as a powder using an electrostatic technique. The process hitherto has been applied primarily to aluminium powder, but other metallic powders such as nickel and stainless steel are suitable.

It is known that powdered metal can be sprayed .from a commercially-available spray gun, such as the electrostatic spray gun, known as the Strajet, or it can be blown across or otherwise introduced to a high voltage field, which is capable of inducing a corona effect between charged electrodes and the metal strip. The application of high voltage (9C-150 kv.) between the gun and the metal strip has the effect of electrostatically charging the particles of metal powder on leaving the gun. The charged particles, which repel one another, are attracted to the strip and adhere to the strip under the influence of the electrostatic forces. The degree of adherence of the particles to the strip depends on their ability to retain the electrostatic charge, which ability depends on the resistivity of the particles. Thus particles with a high resistivity e.g. plastic powders, lose their charge very slowly and adhere firmly for a long time. On the other hand metallic powders have a low resistivity, lose their charge very quickly and fall olii` the strip almost immediately. Some metals, however notably aluminium, have an oxide lm on the surface which due to its high resistivity helps to retain the electrostatic charge. Despite the presence of the oxide film it is not sufficient to promote good adhesion and after an aluminium powder is sprayed onto a dry surface, only the very lightest particles remain on the surface so that the coating remains thin and is easily dislodged.

The present invention relates to a method of promoting the adherence of all types of metal powders to conducting surfaces.

We have discovered that when a liquid film of high resistivity is in contact with the strip, the particles of powder striking the iilm retain their charge for much longer periods and a uniform coating, which is not only thicker but is more adherent than that obtained when the powder is sprayed on untreated strip, can be obtained. It is ICC thought that some of the additional adherence is due to the insulating effect of the liquid film and some to the surface tension effect of the liquid film. Indeed even after the film has been removed by evaporation the particles show much greater adherence than previously.

According to the present invention in the manufacture of a coated metal strip in which the coating metal or alloy is applied in powder form by spraying, the surface of the rneta1 strip is wetted prior to application of the surface coating.

The invention is illustrated in the accompanying drawings, in which:

FIGURE 1 is a diagrammatic view of a strip coating and treatment line in accordance with the invention.

FIGURES 2 and 3 are diagrammatic views showing two alternative lforms of coating (nitrogen) chamber for use in the treatment line shown in FIGURE l.

Referring to the drawings, coiled metal strip S is drawn olf a reel or uncoiler 1 whence it passes over a guide sheave 2 into the end compartment 3 of a three-compartment tank 4 and thence through successive compartments 5, 6, in passage between which it passes over and under further guide sheaves as sh-own. From the compartment 6 the strip S is fed to a coating chamber 7.

The three compartments 3, 5 and 6 contain cleaning fluid, rinse liquid and wetting liquid respectively. It will be understood that the last compartment 6 functions as the wetting tank so that the strip S before entering the coating chamber 7 is thoroughly and evenly wetted on both sides.

The strip may be wetted by immersion in a bath containing a liquid having a low surface tension. Alternatively, the chosen liquid may be supplied to the strip through sprays. The process has been successfully carried out with water together with the use of a suitable wetting agent, or by immersion in methyl and ethyl alcohols, but several other types of liquids and solutions have been found suitable.

Any surplus liquid is removed from the strip using a wiper seal or squeegee rolls 15 so that the emergent strip has a thin film of liquid over its entire surface. The effect of the liquid film is to increase the eliiciency of the deposition process and to improve the bond that can be obtained between the metallic powder and the surface of the strip.

In passage through the coating chamber 7 the strip is sprayed with powder in -a field of high potential (9D-150 kv.), whereupon the powder adheres firmly to the Wet strip.

It is desirable to dry the strip following the powder spraying operation and before further treatment, which may be carried out by passing it through a conventional hot air stove 8, or by using an infra-red. furnace. It will be found that the effect of the initial liquid film is to increase the bond between the powder and the strip even after the strip has been completely dried.

The powder coating so applied can subsequently be compacted to a homogeneous layer by passing the coated strip through a rolling mill 10, which, if followed by a heat treatment, will improve the bond between the aluminium and the steel or other metal basev strip.

As shown on leaving the mill 10, the strip is passed through an oven 11 and thence to a recoiler.

The following data relates to a typical example of the method:

Well-oiled, bright rolled mild steel strip of specification En. 2 .010 thick, 6" wide, was fed continuously from an uncoiler at 15 ft./minute into a cleaning or degreasing compartment where the stri-p surfaces were scrubbed with wire brushes whilst subject to jets of an alkaline degreasing liquid. The strip was then passed to a rinse compartment and rinsed in cold water. The strip then was passed to a dip or wetting tank containing a 21/2% solution of hydrochloric acid to neutralise the remaining alkalis. This was followed by a -iinal rinse in cold water containing 1/2% of the dispersing wetting agent sold under the trademark Lissapol NDB, the surplus liquid being removed from the strip by squeegee rolls. Lissapol NDB is a dilution of Lissapol N, the latter being an aqueous solution of nonylphenol ethylene oxide condensate, that is, one of the most recently developed types of surfaceactive agent containing a polyethylene glycol chain, and represented by the formula R-O-C2H4 DOH Whilst still wet the strip was then passed under an electrostatic powder spraying head which was held at 90,000 volts. Atomised aluminium powder in the size range 250 mesh/ dust Was conveyed pneumatically to the head using nitrogen as the conveying gas. The nitrogen usage was 2.4 cu. ft./minute. The aluminium powder was attracted to the strip where it adhered firmly. The strip was then dried by a 6 kw. radiant heater and subjected to a rolling load of 42 tons to compact the aluminum powder into a continuous layer. After rolling, the metal clad strip was recoiled.

The coil of clad strip was then placed in a gas fired semi-muffle furnace and heated to 500 C. and held at that temperature for one hour. On examination it was seen that a continuous coating of aluminium had been obtained which was securely bonded to the base steel. In a cup test in which the 'coated steel was deformed by a steel ball, it was Lfound that the bond was such that the base steel cracked before the coating became delaminated. The coating had a thickness which varied between .0007" and .0001.

Some metallic powders are liable to explode in the atmosphere, especially if it is possible for electric arcing to occur. To overcome this difficulty, according to a further feature of the invention, the powder was conveyed to the discharge nozzle of the spray gun by an inert gas, e.g. nitrogen. To minimise further the risk of explosion, the spraying may be carried out in a nitrogen environment in a sealed chamber. The presence of the inert atmosphere is not essential to the process of deposition but is desirable as a means of reducing the risk of explosion. In the case of aluminium, it has .only been found necessary to convey the powder in an inert atmosphere and the spraying can be carried out in air. A sealed chamber would be necessary if it was desired to spray, say magnesium.

Referring now to FIGURES 2 and 3 the metal strip S passes into the chamber 7 through a self sealing opening 12, -where it is subjected to the `action of a series of Spray guns 13, 13' arranged to spray opposite sides of the strip. While satisfactory results have been obtained using a Wi'per type seal 12 at the ingoing end of the chamber 7, to avoid its use on the coated strip at the outgoing end, which would tend to dislodge the deposited particles, the wiper seal is arranged to co-operate with the surfaces of the rolls of the mill 10. Alternatively the roll mill or part .of it can be installed inside the chamber 7.

Among the advantages of the present invention are (l) Spraying may be carried out while the strip is moved in any direction. Thus, a horizontal pass may be used and this simplifies operation of the plant and gives the cheapest form of plant installation.

(2) The power consumed by the spraying apparatus is extremely small.

(3) The drying unit may be of small capacity since only a relatively small, controlled amount of liquid has to be removed by drying.

(4) No recoveryv unit is necessary for the liquid because of the extremely small quantities of liquid involved.

What is claimed is:

1. A method of coating at least one surface of metal strip with a different metal comprising (i) wetting the surface to be coated with a liquid of high resistivity and low surface tension;

(ii) applying to said surface while the later is wet a powder containing electrostatically charged powder particles of said different metal;

(iii) maintaining the strip at a predetermined potential to provide electrostatic attraction between the particles and the strip;

(iv) drying the surface and powder; and

(v) compacting said particles against said surface.

2. A method of coating at least one surface of metal strip with aluminum comprising (i) wetting the surface to be coated with a liquid of high resistivity and low surface tension;

(ii) applying to said surface while said surface is wet a powder containing electrostatically charged powder particles of aluminum;

(iii) maintaining the strip at a predetermined potential to provide electrostatic attraction between the particles and the strip;

(iv) removing moisture from the surface and the powder applied to said surface;

(v) compacting said particles against said surface; and

(vi) heating the strip to approximately 500 C. to bond said aluminum to the surface of said strip.

3. A method of coating at least one surface of metal strip with a different metal comprising:

(i) wetting the surface to be coated with a liquid of high resistivity and low surface tension;

(ii) applying .to said surface while the latter is wet a powder containing electrostatically charged particles of another metal;

(iii) maintaining the strip at a predetermined potential to provide electrostatic attraction between the particles and the strip;

(iv) drying the surface and powder applied thereto;

('v) compacting said particles against said surface; and

(vi) heating the coated strip to bond said particles to said surface of said strip.

4. The method set forth in claim 3 wherein said surface is wetted by spraying it with a liquid containing a wetting agent.

5. The Amethod set forth in claim 3 wherein said surface is wetted by immersing said strip in a liquid containing a wetting agent.

6. A method of coating at least one surface of metal strip with a different metal lcomprising (i) wetting `the surface to be coated with a liquid of high resistivity and low surface tension;

(ii) applying to said surface while the latter is wet a powder containing electrostatically charged powder particles of said different metal;

(iii) maintaining 4the strip at a predetermined potential to provide electrostatic attraction between the particles and the strip;

(iv) heat treatin-g the strip and powder sufficiently to remove residual moisture;

(v) compacting said particles against said surface; and

(vi) heating the coated strip to bond said particles to said surface of said strip.

7. A method of coating at least one surface of metal strip with a different metal comprising (i) wetting the surface to be coated with a liquid of high resistivity and low surface tension;

(ii) applying to said surface in ian inert atmosphere and while said surface is wet a powder containing electrostatically charged powder particles of said different metal;

(iii) maintaining the strip at a predetermined potential to provide electrostatic attraction between the particles and the strip;

(iv) removing moisture from the powder applied to said surface;

5 6 (v) compacting said particles against said surface; and 10. The method set forth in claim 9 in which the spray- (vi) heating the coated strip to bond said particles to ing of said powder is performed in an inert atmosphere.

said surface of said strip. 11. The method set forth in claim 8 wherein said coated 8. A method of coating both surfaces of metal strip strip is heated to a temperature below the fusion point of with aditerent metal comprising: 5 said different metal to bond the particles of the different (i) wetting both surfaces of said strip; with a liquid metal to the strip.

of high resistivity and low surface tension; References Cited it vplf achnfidg rft-h'ythfhlat UNITED STATES PATENTE s e e co a1 n ee 1c arg powder particles of said different metal; 10 gy'm /1933 McCulloch 11./22 X (iii) maintaining the strip at a predetermined potential zgwgo 1 195 Allen 117-93'4 X to provide electrostatic attraction between the parti- 8 3/195 Heck 117`31 X des and the Strip; 2,990,293 6/ 1961 Toulmm L17-93.4 X (iv) compacting said particles against said surfaces; and FOREIGN PATENTS (v) heatmg the coated strip to bon-d said particles to 15 1,023,224 3/1966 Great Britain said surfaces. 9. The method set forth in claim 8 wherein said powder RALPH S, KENDALL primary Examie, is sprayed onto said surfaces with the metal particles of said powder electrostatically charged. 

